  One of the most important problems in nanoelectronic device theory is to
estimate how fast or how slow a quantum device can turn on/off a current. For
an arbitrary noninteracting phase-coherent device scattering region connected
to the outside world by leads, we have derived an exact solution for the
nonequilibrium, nonlinear, and time-dependent current driven by both up- and
down-step pulsed voltages. Our analysis is based on the Keldysh nonequilibrium
Green's functions formalism where the electronic structure of the leads as well
as the scattering region are treated on an equal footing. A model calculation
for a quantum dot with a Lorentzian linewidth function shows that the
time-dependent current dynamics display interesting finite-bandwidth effects
not captured by the commonly used wideband approximation.
