We have performed low-density (< 10(5) cm(-1)) time-resolved photoluminescence experiments that probe the relaxation of photo-excited carriers in a quantum wire. Our results show that carrier-carrier collisions are dominant at very short times both for intra- and for inter-subband scattering as only expected if non-Markovian effects are taken into account. (C) 1999 Elsevier Science B.V. All rights reserved.