Weak localization or coherent backscattering is a general effect for waves scattered by a disordered medium. It is an interference effect that survives over length scales much larger than the mean free path, and arises from the time reversal symmetry of the wavelets scattered in the backscattering direction. In the exact backscattering direction, counter propagating wavelets will interfere constructively, while sufficiently far away from this direction phase differences wash out the interference effect. The result is a cone of enhanced backscattering which has a width of the order of l/l, where l is the wavelength and l is the (transport) mean free path.

Optical Society of America
R.R. Alfano

Wiersma, D. S., van Albada, M. P., & Lagendijk, A. (1994). Accurate analysis of coherent backscattering revealing recurrent scattering of light in disordered media. In R. R. Alfano (Ed.), OSA Proceedings on Advances in Optical Imaging and Photon Migration ; vol. 21 (pp. 53–57).