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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=2Δn, for $n=2$ and $n=3$ in excellent agreement with theories of multiparticle tunneling and multiple Andreev reflection.
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Post et al. (1994) studied this question.
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