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Weak antilocalization of a two-dimensional electron gas formed at a In0.53Ga0.47As/InP heterointerface was studied. The Fermi level was varied from below, to above, the energy minimum of the second subband. A model for quantum coherence with two conducting subbands and fast intersubband scattering was used to extract the characteristic phase and spin decoherence rates from experimental magnetoresistance data. Taking into account the spatial inhomogeneity of the energy associated with the subband minimum, the first and second subband decoherence contributions were separated. It was shown that phase decoherence in the second subband is much faster than in the first subband and it decreases with increasing occupation of the second subband. By contrast, spin dephasing due to scattering in the second subband and intersubband scattering does not play a noticeable role.