It is shown analytically and by numerical simulation that the angular distribution of Andreev reflection by a disordered normal-metal-superconductor junction has a narrow peak at the angle of incidence. The peak is higher than the well-known coherent backscattering peak in the normal state, by a large factor G/G0 (where G is the conductance of the junction and G0 = 2e 2 /h). The enhanced backscattering can be detected by means of ballistic point contacts.PACS numbers: 74.80. Fp, 72.15.Rn, 73.50.Jt, 74.50.+r -cond-mat/9501003 Coherent backscattering is a fundamental effect of time-reversal symmetry on the reflection of electrons by a disordered metal [1,2]. The angular reflection distribution has a narrow peak at the angle of incidence, due to the constructive interference of time-reversed sequences of multiple scattering events. At zero temperature, the peak is twice as high as the background. Coherent backscattering manifests itself in a transport experiment as a small negative correction of order G 0 = 2e 2 /h to the average conductance G of the metal (weak localization [3]). Here we report the theoretical prediction, supported by numerical simulations, of a giant enhancement of the backscattering peak if the normal metal (N) is in contact with a superconductor (S). At the NS interface an electron incident from N is reflected either as an electron (normal reflection) or as a hole (Andreev reflection). Both scattering processes contribute to the backscattering peak. Normal reflection contributes a factor of two. In contrast, we find that Andreev reflection contributes a factor G/G 0 , which is ≫ 1.If the backscattering peak in an NS junction is so large, why has it not been noticed before in a transport experiment? The reason is a cancellation in the integrated angular reflection distribution which effectively eliminates the contribution from enhanced backscattering to the conductance of the NS junction. However, this cancellation does not occur if one uses a ballistic point contact to inject the current into the junction. We discuss two configurations, both of which show an excess conductance due to enhanced backscattering which is a factor G/G 0 greater than the weak-localization correction.We consider a disordered normal-metal conductor (length L, width W , mean free path l, with N propagating transverse modes at the Fermi energy E F ) which is connected at one end to a superconductor (see inset of Fig. 1). An electron (energy E F ) incident from the opposite end in mode m is reflected into some other mode n, either as an electron or as a hole, with probability amplitudes r ee nm and r he nm , respectively. The N × N matrices r ee and r he are given by [4]The s ij 's are submatrices of the scattering matrix S of the disordered normal region,where u, v, u ′ , v ′ are N × N unitary matrices, R = 1 − T , and T is a diagonal matrix with the transmission eigenvalues T 1 , T 2 , . . . T N on the diagonal. We first consider zero magnetic field (B = 0). Timereversal symmetry then requires that S is a symmetric ma...