Diffusive transport and weak antilocalization (WAL). (a) Illustration of a time-reversed pair of electron trajectories on a self-crossing loop. Destructive and constructive interferences between them lead to weak antilocalization and weak localization (WL), respectively. (b) Sketch of the band diagram of a topological insulator with Fermi energy E_F located in the bulk conduction band. When E_F is in the shaded regions, scatterings between the bulk and the surface states make the sample behave like a single-channel system in the phase coherent transport. (c) Extracted α values from fitting the low field magnetoconductivity data to the simplified HLN equation [Eq. (1)], which are nearly independent of the sheet electron density n_e. (d) Prefactor α plotted as a function of mobility μ. Panels (c) and (d) are adapted from Ref. [20].