Rabi oscillations may be viewed as an interference phenomenon due to a coherent superposition of different quantum paths, as in Young's two-slit experiment. The inclusion of the atomic external variables causes a non-dissipative damping of the Rabi oscillations. More generally, the atomic translational dynamics induces damping in the correlation functions which describe non-classical behaviors of the field and internal atomic variables, leading to the separability of these two subsystems. We discuss on the possibility of interpreting this intrinsic decoherence as a "which-way" information effect and we apply to this case a quantitative analysis of the complementarity relation as introduced by Englert (Phys. Rev. Lett. 77 (1996) 2154).