  We present a theory of weak localization (WL) in the presence of generic
spin-dependent fields, including any type of spin-orbit coupling, Zeeman
fields, and non-homogeneous magnetic textures. We go beyond the usual diffusive
approximation, considering systems with short-range disorder of arbitrary
strength, and obtain a compact expression for the weak localization (WL)
correction to the conductivity in terms of the singlet-triplet polarization
operator in momentum space. The latter can be directly related to the solution
of the quasiclassical Eilenberger equation for superconducting systems. This
formulation presents an intuitive framework to explore how the interplay of
various spin-dependent fields drives weak (anti) localization. We apply our
results to study in-plane magnetoconductivity in systems with spin-orbit
coupling, and in newly discovered altermagnets. Our results enable
straightforward calculation of the WL conductivity at arbitrary disorder
strength, which can be particularly useful for interpreting experiments on
high-mobility samples.
