  Quantum pumping holds great potential for future applications in micro- and
nanotechnology. Its main feature, dissipationless charge transport, is
theoretically possible via several different mechanisms. However, since no
unambiguous verification has been demonstrated experimentally, the question of
finding a viable mechanism for pumping remains open. Here we study quantum
pumping in an one dimensional electron waveguide with a single time-dependent
barrier. The quantum pumping of electrons using a potential barrier whose
height and position are harmonically varied is analyzed analytically and by
numerically solving the time-dependent Schr{\"o}dinger equation. The pumped
charge is modeled analytically by including two contributions in linear
response theory. First, the scattering of electrons off a potential moving
slowly through matter-waves gives a contribution independent of the
translational velocity of the potential. Second, Doppler-shifted scattering
events give rise to a velocity dependent contribution, which is found in
general to be small in comparison with the first one. The relative phase
between the oscillations of the height and position is found to be the factor
that determines to what extent either contribution is present.
