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Using a mechanically controllable breakjunction we study superconducting tunnel junctions of atom size. The vacuum barrier is adjusted by controlling the distance between the electrodes to give barrier transparencies ranging over 2 orders of magnitude. We observe a systematic variation of the subgap current steps at eV=2n, for n=2 and n=3 in excellent agreement with theories of multiparticle tunneling and multiple Andreev reflection.
Post et al. (Mon,) studied this question.
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