  We measure simultaneously the in-plane electron g-factor and spin relaxation
rate in a series of undoped inversion-asymmetric (001)-oriented GaAs/AlGaAs
quantum wells by spin-quantum beat spectroscopy. In combination the two
quantities reveal the absolute values of both the Rashba and the Dresselhaus
coefficients and prove that the Rashba coefficient can be negligibly small
despite huge conduction band potential gradients which break the inversion
symmetry. The negligible Rashba coefficient is a consequence of the
'isomorphism' of conduction and valence band potentials in quantum systems
where the asymmetry is solely produced by alloy variations.
