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...

In Josephson junctions, a supercurrent across a nonsuperconducting weak link is carried by electron-hole bound states. Because of the helical spin texture of nondegenerate topological surface states, gapless bound states are established in junctions with topological weak link. These have a characteristic 4π-periodic current phase relation (CΦR) that leads to twice the conventional Shapiro step separation voltage in radio frequency–dependent measurements. In this context, we identify an attenuated first Shapiro step in (Bi0.06Sb0.94)2Te3 (BST) Josephson junctions with AlOx capping layer. We further investigate junctions on narrow, selectively deposited BST nanoribbons, where surface charges are confined to the perimeter of the nanoribbon. Within these junctions, previously identified signatures of gapless bound states are absent. Because of confinement, transverse momentum sub-bands are quantized and a topological gap opening is observed. Surface states within these quantized sub-bands are spin degenerate, which evokes bound states of conventional 2π-periodic CΦR within the BST nanoribbon weak link.