We analyze the effect of weak residual disorder in microstructures defined on high-mobility heterojunctions, where the classical electron motion is ballistic. We parameterize the disorder by its correlation length { xi} and the elastic mean free path { l}, which can be estimated from microscopic models. For the experimentally relevant case in which { xi} is not negligible with respect to the size of the microstructure, we present a perturbative semiclassical approach demonstrating that the reduction due to disorder of a two-point Green- function observable, like the average magnetic susceptibility, is intrinsically different from the bulk (power law depending on { l} and { xi} instead of an exponential damping governed uniquely by { l}). The very weak damping that we obtain supports the use of clean (no disorder) models for the ballistic regime. Our analytical findings are confirmed by exact numerical calculations.