Evolution of the S² parameter for all models. Panel (a) shows the evolution of S² with respect to the physical time in the simulations. All models show different degrees of mixing at most times. At t = 12 Gyr (vertical line), model N2M is the least mixed, while model N05M is the most mixed in phase space. Panel (b) shows the evolution of S² as a function of the number of half-mass relaxation times. The symbols show the value of S² at 12 Gyr and the respective number of relaxation times.