Interference of bulk and surface scattering processes is analyzed for ultra
thin systems with quantized motion in the direction perpendicular to the wa
lls. The effective relaxation time for scattering by random bulk and surfac
e inhomogeneities is calculated, when possible analytically, beyond the Mat
thiessen's rule. The applications to the size effect, weak localization, tr
ansport along the walls, and boundary slip are discussed. (C) 2000 Elsevier
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