  We present an analytical study of the proximity effect in
ferromagnet/superconductor (F/S) heterostructures, allowing for an arbitrary
magnetic exchange energy as well as arbitrary impurity and spin-flip scattering
rates within a quasiclassical approach. While previous studies mainly have
focused on the clean or dirty limits, our results grant access to the regime of
intermediate impurity concentrations, thus allowing us to probe the crossover
from the clean to dirty limit. We find that in the crossover regime, all
possible symmetry correlations of the proximity-induced anomalous Green's
function are induced in the ferromagnet. We also point out that the local
density of states oscillates spatially, not only for an F/S bilayer, but also
for a normal/superconductor (N/S) bilayer in the diffusive limit, a fact which
appears to have gone unnoticed in the literature. Within the weak-proximity
effect regime, we present compact analytical expressions valid for arbitrary
exchange fields and impurity scattering rates for \textit{i)} the local density
of states in an F/S bilayer, \textit{ii)} the Josephson current in an S/F/S
junction, and \textit{iii)} the critical temperature in an F/S/F multilayer.
For all cases, we study in particular the crossover regime between diffusive
and ballistic motion. Our results may be useful for analyzing experimental data
in cases when the dirty limit is not fully reached, thus invalidating the use
of the Usadel equation.
