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 mu<10-20 000 cm(2)/Vs. To achieve mobilities approaching 10(5) cm(2)/Vs we have used strained Si:SiGe quantum wells grown on substrates tilted away from the (001) normal by 0 degrees, 2 degrees, 4 degrees, 6 degrees, and 10 degrees. 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 degrees and 4 degrees 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 degrees and 4 degrees samples to the presence of a minigap. For the 6 degrees and 10 degrees samples the minigap is higher than the Fermi energy and is not expected to influence the transport properties.