Using two independent magnetotransport experiments, i.e., thermal activation and the coincidence method in tilted fields, we determine the g factor in a two-dimensional electron system in a 4-nm-wide InAs quantum well. From these independent techniques we deduce consistently an absolute value |gexp|≅6. This is considerably smaller if compared to |g|=14.8 for bulk InAs. Nonparabolicity in InAs cannot fully explain the reduced g factor. We argue that the penetration of the wave function into the In0.75Ga0.25As barriers and into the In0.75Al0.25As spacer layer plays an additional role. © 2003 American Institute of Physics.