  We study electron propagation through a random array of rare, opaque and
large (compared the de Broglie wavelength of electrons) scatterers. It is shown
that for any convex scatterer the ratio of the transport to quantum lifetimes
\eta=\tau_{tr}/\tau_{tot}$ does not depend on the shape of the scatterer but
only on whether scattering is specular or diffuse and on the spatial
dimensionality (D). In particular, for specular scattering, \eta is a universal
constant determined only by the dimensionality of the system: \eta = 2 for D =
3 and \eta = 3/2 for D = 2. The crossover between classical and quantum regimes
of scattering is discussed.
