Weak localization in GaMnAs: evidence of impurity band transport

We report the observation of negative magnetoresistance in the ferromagnetic semiconductor GaMnAs at low temperatures ($T<3$ K) and low magnetic fields ($0< B <20$ mT). We attribute this effect to weak localization. Observation of weak localization provides a strong evidence of impurity band transport in these materials, since for valence band transport one expects either weak anti-localization due to strong spin-orbit interactions or total suppression of interference by intrinsic magnetization. In addition to the weak localization, we observe Altshuler-Aronov electron-electron interactions effect in this material.