  We present a general formulation 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 non-interacting
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.
