  We study quantum transport of an interacting Bose-Einstein condensate in a
two-dimensional disorder potential. In the limit of vanishing atom-atom
interaction, a sharp cone in the angle-resolved density of the scattered matter
wave is observed, arising from constructive interference between amplitudes
propagating along reversed scattering paths. Weak interaction transforms this
coherent backscattering peak into a pronounced dip, indicating destructive
instead of constructive interference. We reproduce this result, obtained from
the numerical integration of the Gross-Pitaevskii equation, by a diagrammatic
theory of weak localization in presence of a nonlinearity.
