The magnetotransport properties in pseudomorphic GaAs/InGaAs/GaAs and AlGaAs/InGaAs/GaAs quantum wells with a Si &dgr;‐doped layer in the barrier close to the quantum well have been investigated. In the GaAs/InGaAs/GaAs system, a giant negative magnetoresistance is observed in the parallel magnetic field configuration. The effect is less pronounced if the doping is performed in the AlGaAs barrier. We attribute this giant negative magnetoresistance to the reduction of an additional impurity scattering process between two‐dimensional states in the quantum well and inhomogeneously distributed donor states in the &dgr;‐doped layer. The experimental results in the GaAs/InGaAs/GaAs system are found to be in excellent agreement with a model that takes into account an oscillation between two‐dimensional states and the &dgr;‐doped layer including an additional momentum in parallel magnetic fields. ©1995 American Institute of Physics.