Disorder induced quantized conductance with fractional value and universal conductance fluctuation in three-dimensional topological insulators

We report a theoretical investigation on the conductance and its fluctuation of three-dimensional topological insulators (3D TI) in \textit{Bi}2\textit{Se}3 and \textit{Sb}2\textit{Te}3 in the presence of disorders. Extensive numerical simulations are carried out. We find that in the diffusive regime the conductance is quantized with fractional value. Importantly, the conductance fluctuation is also quantized with a universal value. For 3D TI connected by two terminals, three independent conductances $G$\textit{zz}, $G$\textit{xx} and $G$\textit{zx} are identified where z is the normal direction of quintuple layer of 3D TI. The quantized conductance are found to be $\left\langle {G_{zz} } \right\rangle $ = 1, $\left\langle {G_{xx} } \right\rangle $ = 4/3 and $\left\langle {G_{zx} } \right\rangle $ = 6/5 with corresponding quantized conductance fluctuation 0.54, 0.47, and 0.50. The quantization of average conductance and its fluctuation can be understood by theory of mode mixing. The experimental realization that can observe the quantization of average conductance is discussed.