Effect of the randomness on the electron transport in normal metal/insulator/conventional superconductor junctions

Tunneling conductance at zero voltage in diffusive normal metal (DN)/insulator/s-wave superconductor junctions is calculated for various situations based on the quasiclassical (QC) theory. In QC theory, the generalized boundary condition introduced by Nazarov [Superlattices and Microstructures 25 1221 (1999)] is applied, where ballistic theory by Blonder Tinkham and Klapwijk and diffusive theory by Volkov Zaitsev and Klapwijk based on the boundary condition by Kupriyanov and Lukichev are naturally reproduced. It is shown that with the increase of the magnitude of resistance of DN (RD), normalized tunneling conductance is enhanced (suppressed) for low (high) transparent barrier. When the length of DN is larger than that of localization length, tunneling conductance obtained by numerical simulation deviates from that by QC theory