  We investigate the current-voltage characteristics of high-transparency
superconductor-insulator-normal metal (SIN) junctions with the specific tunnel
resistance below 30 kOhm per square micron. The junctions were fabricated from
different superconducting and normal conducting materials, including Nb, Al,
AuPd and Cu. The subgap leakage currents were found to be appreciably larger
than those given by the standard tunnelling model. We explain our results using
the model of two-electron tunnelling in the coherent diffusive transport
regime. We demonstrate that even in the high-transparency SIN-junctions, a
noticeable reduction of the subgap current can be achieved by splitting a
junction into several submicron sub-junctions. These structures can be used as
nonlinear low-noise shunts in Rapid-Single-Flux-Quantum (RSFQ) circuitry for
controlling Josephson qubits.
