  The problem of a beam of quantum particles falling through a diffractive
screen is studied. The solutions for single and double slits are obtained
explicitly when the potential is approximated by a linear function. It is found
that the resulting patterns depend on a quasi-time $\tau$ given by a function
of the coordinate along the propagation axis in a classical combination
$z_0-t^2 F/m$, while the diffraction effects along transverse axes are due
solely to $m/\hbar$. The consequences on the precision at which the equivalence
principle can be tested are discussed. Realizations with ultra cold neutrons,
Bose-Einstein condensates and molecular beams are proposed.
