Quantization of conductance minimum and index theorem

We discuss the minimum value of the zero-bias differential conductance G sub(min) in a junction consisting of a normal metal and a nodal superconductor preserving time-reversal symmetry. Using the quasiclassical Green function method, we show that G sub(min) is quantized at (4e super(2)/h )N sub(ZES) in the limit of strong impurity scatterings in the normal metal at the zero temperature. The integer N sub(ZES) represents the number of perfect transmission channels through the junction. An analysis of the chiral symmetry of the Hamiltonian indicates that N sub(ZES) corresponds to the Atiyah-Singer index in mathematics.