A topological insulator (TI), which is topologically distinct from an ordinary insulator, is a material with a bulk electronic excitation gap generated by the spin-orbit interaction. This distinction, characterized by a Z2 topological invariant, necessitates the existence of gapless electronic states on the sample boundary. In two dimensions (2D), the TI is a quantum spin Hall insulator. A strong TI is expected to have surface states whose Fermi surfaces enclose an odd number of Dirac points. This defines a topological metal surface phase that is predicted to have novel electronic properties. The first TI to be discovered was the alloy Bi1−xSbx, the unusual surface bands of which were mapped in an angle-resolved photoemission spectroscopy (ARPES) experiment [1]. The semiconducting alloy Bi1−xSbx is a strong TI owing to the inversion symmetry of bulk crystalline Bi and Sb. We have investigated the temperature dependence of resistance of TI Bi0.83Sb0.17 nanowires with varying diameters (75 nm ≤ d ≤ 1100 nm), obtained by radio frequency casting in a glass capillary. Owing to the quantum size effect, the energy gap ΔE with decreasing diameter of the nanowires (d from 1100 nm down to 75 nm) increases as ΔE ~ 1/d (for diameter d = 1.1 µm and d = 75 nm, ΔE = 21 and 45 meV, respectively). From the linear dependence of nanowire conductance on nanowire diameter at T = 4.2 K (Fig. 1a), we calculated the square resistance Rsq of the surface states of the nanowires to be only 70 Ohm [2]. In Bi0.83Sb0.17 nanowires with d ≤ 100 nm we discovered the Aharonov–Bohm oscillations of the longitudinal magnetoresistance with periods of one and half of flux quantum (Fig 1b).