  Localized scattering phenomena may result in the formation of stationary
matter waves originating from a compact region in physical space.
Mathematically, such waves are advantageously expressed in terms of quantum
sources that are introduced into the Schr\"odinger equation. The source
formalism yields direct access to the scattering wave function, particle
distribution, and total current. As an example, we study emission from
three-dimensional Gaussian sources into a homogeneous force field. This model
describes the behaviour of an atom laser supplied by an ideal Bose-Einstein
condensate under the influence of gravity. We predict a strong dependence of
the beam profile on the condensate size and the presence of interference
phenomena recently observed in photodetachment experiments.
