Single particle relaxation time and the scattering time of the 2DEG in modulation doped GaAs/AlGaAs heterostructure are measured by Ladau respectively, for samples with spacer layer thickness 1.5-500 nm. The results are compared with the recent calculations of Das Sarma and Stern and excellent agreement is obtained. When the excited subband is occupied under persistent photoconductive (PPC) conditions, the relaxation time for the excited subband was found to be nearly three times longer than that for the ground subbands. The results are interpreted in terms of screened remote impurity scattering. © 1987.