We present experimental data on the ballistic weak localisation in square cavities filled with antidot arrays. The weak localisation peak is cusp-like at mK temperatures and clearly deviates from a Lorentzian line shape, which is the theoretical prediction for a classical chaotic system. Although the ideal system, a Sinai billiard, is in principle purely chaotic, directly reflected, non-chaotic trajectories lead to the behaviour of a mixed system in experiment. We demonstrate this by detailed semiclassical analysis. In comparison, empty cavities of equal geometry show a sharper peak in the form of a Lorentzian. Here, imperfections of the experimental structure (boundary roughness, small- angle scattering) seem to change the dynamics of the nominally regular system to chaotic, contrary to the antidot case.