In this paper we present the observation of ballistic transport in strained silicon cavities defined by etching on a silicon germanium heterostructure, demonstrated by magnetic focusing of conductance of the cavity at T = 50 mK. Numerical simulations, based on a novel approach which allows to include an arbitrary degree of decoherence in mesoscopic transport, show that magnetoconductance features can be related to the semiclassical orbits by means of the local density of states in the cavity.