Universal conductance fluctuations in a multi-subband quantum well

We have investigated mesoscopic conductance fluctuations in a GaAs quantum well with five electrically quantized two-dimensional (2D) subbands occupied. In contrast to the case of a single occupied subband, where the fluctuations depend only on the component of magnetic field perpendicular to the plane of the electrons, in our multi-subband structures the conductance fluctuations occur even in a magnetic field parallel to the plane. We attribute the fluctuations to rapid electron-impurity scattering between different 2D subbands which leads to quasi-3D motion of electrons within the quantum well. We are able to confirm the 3D character of the fluctuations by correlations between the magnetoresistances observed when the magnetic field is applied at various angles relative to the plane of the quantum well.