  Wave-particle duality finds a natural application for electrons or light
propagating in disordered media where coherent corrections to transport are
given by two-wave interference. For scatterers with internal degrees of
freedom, these corrections are observed to be much smaller than would be
expected for structureless scatterers. By examining the basic example of the
scattering of one photon by two spin-1/2 atoms--a case-study for coherent
backscattering--we demonstrate that the loss of interference strength is
associated with which-path information stored by the scattering atoms.
