Positive magnetoconductivity in the strong localization regime. (a) Gate voltage dependence of zero-field sheet resistivity ρ_xx of a 2 nm thick BST film. (b) Magnetoconductivity curves of the same sample for three representative ρ_xx values, which are marked as points A–C in panel (a). (c) Magnetoconductivity curves of the 2 nm BST thin film in the variable range hopping regime at selected temperatures. The sheet resistivity is ρ_xx = 2.3 MΩ at 1.6 K. Panels (a)–(c) are adapted from Ref. [40]. (d) Magnetoconductivity curves of a 3 nm thick Bi₂Se₃ sample for a set of gate voltages. The inset shows the gate voltage dependence of ρ_xx.