  Coherent backscattering is a multiple scattering interference effect which
enhances the diffuse reflection off a disordered sample in the backward
direction. Classically, the enhanced intensity is twice the average background
under well chosen experimental conditions. We show how the quantum internal
structure of atomic scatterers leads to a significantly smaller enhancement.
Theoretical results for double scattering in the weak localization regime are
presented which confirm recent experimental observations.
