  Simulations based on experimental data obtained from multifragmenting
quasifused nuclei produced in central 129Xe + natSn collisions have been used
to deduce event by event freeze-out properties on the thermal excitation energy
range 4-12 AMeV. From these properties and temperatures deduced from proton
transverse momentum fluctuations constrained caloric curves have been built. At
constant average volumes caloric curves exhibit a monotonous behaviour whereas
for constrained pressures a backbending is observed. Such results support the
existence of a first order phase transition for hot nuclei.
