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A general formulation is presented of the nonlinear, time-dependent current through a small interacting region, where electron energies are changed by time-dependent voltages. An exact solution is obtained for the noninteracting case when the elastic coupling to the leads is independent of energy. Temporal phase coherence in a double-barrier tunneling structure produces ``ringing'' in the response of the current to a voltage pulse, which can be observed experimentally in the dc current by varying the pulse length in a train of voltage pulses. The nonlinear current due to an ac bias also shows complex time dependence.
Wingreen et al. (Wed,) studied this question.