  We utilize a time-periodic ratchet-like potential modulation imposed onto a
two-dimensional electron system inside a GaAs/Al$_x$Ga$_{1-x}$As
heterostructure to evoke a net dc pumping current. The modulation is induced by
two sets of interdigitated gates, interlacing off center, which can be
independently addressed. When the transducers are driven by two identical but
phase-shifted ac signals, a lateral dc pumping current $I(\phi)$ results, which
strongly depends on both, the phase shift $\phi$ and the waveform $V(t)$ of the
imposed gate voltages. We find that for different periodic signals, the phase
dependence $I(\phi)$ closely resembles $V(t)$. A simple linear model of
adiabatic pumping in two-dimensional electron systems is presented, which
reproduces well our experimental findings.
