Sketch of the gapped helical surface states of TIs and the predicted crossover from the WAL to the WL when the ratio Δ/E is varied from 0 to 0.999. (b) Prefactor α obtained from the fit to the simplified HLN equation [see Eq. (1)], which is plotted as a function of the sheet resistivity ρ_xx for four gate-tunable ultrathin Bi₂Se₃ or (Bi,Sb)₂Te₃ films. The data extracted from the experiment are shown in symbols, and the line is a theoretical curve that takes the disorder effect into account.^[70] All of the raw data were taken at T = 1.6 K. Panel (a) is adapted from Ref. [37] and panel (b) from Ref. [40].