ith their experimental verification in 2007, topological insulators (TIs) render a new and fascinating class of materials 1 . A band inversion in the bulk of three-dimensional (3D) TIs creates a 2D metallic subspace at the physical surface of these 3D crystals. The charge carriers of the 2D metal (Dirac electrons) have their spin locked to the momentum, which leads to a topological protection of the subspace 2-4 . This intrinsic quantumspin texture enables the realization of novel technologies, which range from spintronics to quantum computing. Particularly in combination with superconductors (S), TIs promise new quantum devices. Networks of TI nanostructures in proximity to superconductive islands have been predicted to host non-Abelian Majorana modes at the ends and at the crossing points of the networks [5][6][7][8] . Braiding of these elusive modes, that is, exchanging the position of Majorana modes in a 2D plane (Supplementary Fig. 2), resembles topologically protected quantum operations in the Majorana platform. Topological quantum bits (qubits), which use Majorana modes 9,10 to store and process quantum information, are expected to compute fault tolerantly with minimal need for error correction [11][12][13][14] .Topological qubits require high-quality (multi-terminal) Josephson junctions (JJs) 12,15,16 . The simplest type of such a JJ is a two-terminal S-TI-S device (Fig. 1). The Josephson effect 17 allows for an electrical current to conduct dissipationlessly across a lateral junction of two close-by superconductive electrodes separated by a weak link of non-superconductive material. In conventional lateral JJs, the supercurrent is mediated by Andreev bound states (ABS), which effectively transport Cooper pairs across the weak link 18 . In S-TI-S junctions the Dirac system forms a weak link. The quantum spin texture of the Dirac system causes an additional transport channel, known as Majorana bound states (MBS), which adds to conventional ABS 19 . In contrast to ABS, MBS facilitate single-electron transport across the weak link 20 . The contribution of MBS to a supercurrent can be detected via Shapiro response measurements 19,[21][22][23][24] . MBS manifest themselves by a suppression of odd Shapiro steps in low-temperature transport experiments under radio frequency (RF) radiation, due to their 4π-periodic energy-phase dependency 25 .To create and preserve MBS in S-TI-S junctions, the Dirac system in between the superconductive electrodes needs to be conserved (Fig. 1b). Surface oxidation 26,27 and reactions with water molecules at ambient conditions 28 can lead to additional non-topological states at the surface of (Bi,Sb)-based TIs. These superimpose locally with the Dirac system, and thus allow for additional scattering events that could destroy the MBS. To avoid surface degradation in (Bi,Sb)-based TIs, an in situ deposited protective AlO x capping layer on top of the topological surface is often employed 29,30 . Although such capping layers protect the topological surface states for ex situ fabricat...

Conventional 3D TIs are the van der Waals bound, binary compounds such as Bi 2 Te 3 , [10] Bi 2 Se 3 , [11,12] Sb 2 Te 3 , [13] as well as alloys of these elements. [14,15] For a quasi 1D nanowire, due to the inclusion of the Berryphase of a particle traversing the perimeter of the nanowire, the surface band structure is determined to be gapped. [16-18] By applying a magnetic flux parallel to the nanowire axis Φ = ±Φ 0 /2, where Φ 0 = h/e, this gap will be closed. The non-degenerate, topologically protected linear surface bands will re-emerge periodically with a period of a full flux quantum Φ = (n + 1/2) Φ 0 threading the wire. The observation of such periodic Aharonov-Bohm (AB) oscillations has previously been reported. [19-24] Making use of a selective area growth approach, nanowires of aforementioned materials can as well be deposited by molecular beam epitaxy (MBE). [25-28] These nanowires have a rather rectangular cross-section and are therefore referred to as nano ribbons. As a scalable bottom-up approach, these selectively grown nanoribbons are beneficial for desired Majorana surface architectures. [29] However, MBE grown 3D TI compounds usually Quasi-1D nanowires of topological insulators are candidate structures in superconductor hybrid architectures for Majorana fermion based quantum computation schemes. Here, selectively grown Bi 2 Te 3 topological insulator nanoribbons at cryogenic temperatures are investigated. The nanoribbons are defined in deep-etched Si 3 N 4 /SiO 2 nano-trenches on a silicon (111) substrate followed by a selective area growth process via molecular beam epitaxy. The selective area growth is beneficial to the device quality, as no subsequent fabrication needs to be performed to shape the nanoribbons. In the diffusive transport regime of these unintentionally n-doped Bi 2 Te 3 topological insulator nanoribbons, electron trajectories are identified by analyzing angle dependent universal conductance fluctuation spectra. When the sample is tilted from a perpendicular to a parallel magnetic field orientation, these high frequent conductance modulations merge with low frequent Aharonov-Bohm type oscillations originating from the topologically protected surface states along the nanoribbon perimeter. For 500 nm wide Hall bars low frequent Shubnikov-de Haas oscillations are identified in a perpendicular magnetic field orientation. These reveal a topological, high-mobility, 2D transport channel, partially decoupled from the bulk of the material.