  In a four terminal sample microscopic time-reversibility leads to symmetry
relations between resistance measurements where the role of current and voltage
leads are exchanged. These reciprocity relations are a manifestation of general
Onsager-Casimir symmetries in equilibrium systems. We investigate
experimentally the validity of time reversal symmetry in a
$GaAs/Ga_{1-x}Al_xAs$ Hall bar irradiated by an external AC field, at zero
magnetic field. For inhomogeneous AC fields we find strong deviations from
reciprocity relations and show that their origin can be understood from the the
billiard model of a Hall junction. Under homogeneous irradiation the symmetry
is more robust, indicating that time-reversal symmetry is preserved.
