  We develop a new approach to electron transport in mesoscopic systems by
using a particular single-particle basis. Although this basis generates
redundant many-particle amplitudes, it greatly simplifies the treatment. By
using our method for transport of non-interacting electrons, we generalize the
Landauer formula for transient currents and for time-dependent potentials. The
result has a very simple form and clear physical interpretation. As an example,
we apply it to resonant tunneling through a quantum dot where the tunneling
barriers are oscillating in time. We obtain analytical expression for the
time-dependent (ac) resonant current. However, in the adiabatic limit this
expression displays the dc current for zero bias (electron pumping).
