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We have determined the individual transmission coefficients of Al quantum point contacts containing up to six conduction channels. The determination is based on a comparison of the highly nonlinear current-voltage characteristics in the superconducting state with the predictions of the theory for a single channel superconducting contact. We find that at least two channels contribute to the transport even for contacts with conductance lower than the conductance quantum.
Scheer et al. (Mon,) studied this question.