In recent years, with the advent of fine line lithographical methods, molecular beam epitaxy, organometallic vapour phase epitaxy and other experimental techniques, low dimensional structures having quantum confinement in one, two and three dimensions (such as ultrathin films, inversion layers, accumulation layers, quantum well superlattices, quantum well wires, quantum wires superlattices, magneto-size quantizations, and quantum dots) have attracted much attention not only for their potential in uncovering new phenomena in nanoscience and technology, but also for their interesting applications in the areas of quantum effect devices. In ultrathin films, the restriction of the motion of the carriers in the direction normal to the film leads to