Shubnikov-de Haas measurements between 0.06 and 1.6 K have been performed on two modulation-doped n-type heterostructures, and analysed to extract the quantum lifetime. Use of the conventional Dingle formula resulted in deviations from the expected theoretical behaviour above . The corresponding quantum lifetime appeared to increase with temperature in the same range. The data were then re-analysed using a modified expression, in which the thermal damping term was neglected. This gave plots with the correct characteristics, and a quantum lifetime which was approximately constant with temperature, as expected for ionized impurity scattering in a degenerate electron gas. A decrease in the quantum lifetime was observed above , and this is attributed to increased small-angle scattering due to acoustic phonons.