A variation in the saturated phase-breaking time in a ballistic quantum dot is found to occur as the size is varied. This variation differs from that observed earlier, in which a transition from quasi-twodimensional to zero-dimensional behavior was thought to occur. Instead, these results suggest that the saturated phase-breaking rate is governed by a change in the total number of electrons within the dot. At higher temperatures, the phase breaking is governed by coupling to the quantum wire leads, and t f may show the T 22͞3 variation expected for a one-dimensional wire due to the electron-electron interaction.[S0031-9007(99)

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The two-dimensional electron gas formed at the inverted surface of a tilted silicon substrate shows unusual magnetotransport properties due to the presence of a minigap in the density of states. For metal–oxide–semiconductor inversion layers the strong scattering at the interface limits the mobility to values μ<10–20 000 cm2/V s. To achieve mobilities approaching 105 cm2/V s we have used strained Si:SiGe quantum wells grown on substrates tilted away from the (001) normal by 0°, 2°, 4°, 6°, and 10°. Their transport properties have been measured in the temperature range of 20–500 mK. All the samples show strong Shubnikov–de Haas oscillations. For the 2° and 4° samples the envelope of the fast oscillations is modulated by a longer period oscillation at low magnetic fields. We attribute the slow oscillation in the 2° and 4° samples to the presence of a minigap. For the 6° and 10° samples the minigap is higher than the Fermi energy and is not expected to influence the transport properties.

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