Narrow quantum wires of sufficiently high mobility exhibit an anomalous, low-field magnetoresistance peak at a position scaling as the ratio of the Fermi wave vector to the wire width. We investigate this effect varying temperature, electron density, sample geometry, and method of lateral confinement. The data we obtain are consistent with a classical explanation based upon a diffuse component of electron scattering at the edges of the wire. PACS numbers: 73.50.Jt, 73.40.Kp, 73.50.Bk Imposing additional confinement upon a twodimensional electron-gas (2DEG) creates highly conducting one-(ID) and zero-dimensional (OD) devices commonly called "quantum wires" 1 "* 4 and "quantum dots.*' 5 If the potentials are not smooth, however, the resulting electron eigenstates will have short lifetimes and confinement effects may be obscured. In a 2DEG, for example, interface scattering 6,7 can seriously limit the in-plane electron mobility. In this Letter, we consider the analogous problem of electron scattering from the lateral boundaries of quasi-ID wires. 2 Recent experiments have shown that boundary scattering in microfabricated structures is predominantly specular; 2,8 i.e., the probability of specular scattering, /?, 9 is close to unity. This implies that an electron's longitudinal momentum is conserved even after many collisions with the edges. In narrow 2DEG conductors, however, the zero-field sheet resistance is generally seen to markedly increase as the conducting path width decreases and a characteristic feature of transport in such devices is a negative magnetoresistance. 10 " 13 Recently Roukes et al. 4 reported a positive zero-field magnetoresistance in narrow high-mobility wires increasing to an anomalous low-field (B <0.5 T) maximum at a field position, l? m ax, scaling approximately inversely with the wire width, W. Similar features are evident in some of the data of Timp et al. 3 and Ford et al. 14 Here, we describe a possible origin of this unusual anomaly which demonstrates that a small, but significant, amount of the boundary scattering is diffuse (i.e., p < 1).The wires used in this work were defined either by low-energy ion exposure 15 or confinement between split gates. 1 In the former case, areas of a high-mobility 2D EG are protected from the deleterious effects of an ion beam by a narrow mask deposited on the surface. The mask can either be an insulating material 15 or a metal which acts as a self-aligned Schottky-barrier gate so that the carrier density, n, can be varied while keeping a constant conducting width. 16 In either case the ion dose can be optimized so that the electrical width of the wire closely resembles the width of the mask. 17 For split-gate confinement a narrow conducting channel is established in the gap between a pair of reverse-biased
