  We report the observation of a new phenomenon of colossal magnetoresistance
in a 40 nm wide GaAs quantum well in the presence of an external magnetic field
applied parallel to the high-mobility 2D electron layer. In a strong magnetic
field, the magnetoresistance is observed to increase by a factor of ~300 from 0
to 45T without the system undergoing any metal-insulator transition. We discuss
how this colossal magnetoresistance effect cannot be attributed to the spin
degree-of-freedom or localization physics, but most likely emanates from strong
magneto-orbital coupling between the two-dimensional electron gas and the
magnetic field. Our observation is consistent with a field-induced 2D-to-3D
transition in the confined electronic system.
