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We describe measurements on high-current-density Nb-AlOₗ-Nb tunnel junctions and demonstrate that the development of excess subgap current with increasing barrier transparency is the result of multiple Andreev reflections. We present a model for the barrier consistent with measured junction resistance, the high transparency required for multiple traversals, and the observed single-particle and Josephson characteristics. We argue that excess subgap current and subharmonic gap structure in superconducting tunnel junctions are generally likely to be due to pinhole defects in the barriers.
Kleinsasser et al. (1994) studied this question.