Spin–orbit interaction and spin-relaxation mechanisms of a shallow InAs quantum well heterostructure are investigated by magnetoconductance measurements as a function of an applied top-gate voltage. The data are fit using a Iordanskii–Lyanda-Geller–Pikus model and two distinct transport regimes are identified. The spin–orbit interaction splitting energy is extracted from the fits to the data, which also displays two distinct regimes. The different regimes exhibit different spin-scattering mechanisms, the identification of which is of relevance for device platforms of reduced dimensionality which utilize the spin–orbit interaction.