  Four different combinations of $^{96}_{44}$Ru and $^{96}_{40}$Zr nuclei, both
as projectile and target, were investigated at the same bombarding energy of
400$A$ MeV using a $4 \pi$ detector. The degree of isospin mixing between
projectile and target nucleons is mapped across a large portion of the phase
space using two different isospin-tracer observables, the number of measured
protons and the ${\rm t}/^{3}{\rm He}$ yield ratio. The experimental results
show that the global equilibrium is not reached even in the most central
collisions. Quantitative measures of stopping and mixing are extracted from the
data. They are found to exhibit a quite strong sensitivity to the in-medium
(n,n) cross section used in microscopic transport calculations.
