Nonlinear conductance of quantum point contacts

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Abstract
The conductance of ballistic quantum constrictions in a two-dimensional electron gas has been studied experimentally as a function of the applied voltage. Large nonlinearities are found in the current-voltage characteristics. We give a simple model, which explains the main features of the nonlinear conductance. Breakdown of the conductance quantization occurs when the number of occupied one-dimensional subbands becomes unequal for the two velocity directions. A critical voltage is found for the breakdown, which is equal to the subband separation at the Fermi level.