  We formulate a multiple scattering theory of light in media spatially
disordered along two directions and homogeneous along the third one, without
making any paraxial approximation on the wave equation and fully treating the
vector character of light. With this formalism, we calculate the distribution
of transverse momenta of a beam as it evolves along the optical axis, and
unveil a phenomenon not captured by the paraxial equation: a cross-over from a
scalar to a vector regime, visible in the coherent backscattering peak as
polarization gets randomized.
