  Let $(X,d,m)$ be a proper, non-branching, metric measure space. We show
existence and uniqueness of optimal transport maps for cost written as
non-decreasing and strictly convex functions of the distance, provided
$(X,d,m)$ satisfies a new weak property concerning the behavior of $m$ under
the shrinking of sets to points, see Assumption 1. This in particular covers
spaces satisfying the measure contraction property.
  We also prove a stability property for Assumption 1: If $(X,d,m)$ satisfies
Assumption 1 and $\tilde m = g\cdot m$, for some continuous function $g >0$,
then also $(X,d,\tilde m)$ verifies Assumption 1. Since these changes in the
reference measures do not preserve any Ricci type curvature bounds, this shows
that our condition is strictly weaker than measure contraction property.
