  Mn-based nitrides with antiperovskite structures have several properties that
can be utilised for antiferromagnetic spintronics. Their magnetic properties
depend on the structural quality, composition and doping of the cubic
antiperovskite structure. Such nitride thin films are usually produced by
reactive physical vapour deposition, where the deposition rate of N can only be
controlled by the N2 gas flow. We show that the tuning of the N content can be
optimised using low temperature resistivity measurements, which serve as an
indicator of the degree of structural disorder. Several Mn3GaNx films were
prepared by reactive magnetron sputtering under different N2 gas flows. Under
optimised conditions, we obtain films that exhibit a metal-like temperature
dependence, a vanishing logarithmic increase in resistivity towards zero, the
highest resistivity ratio and a lattice contraction of 0.4 % along the growth
direction when heated above that of the N\'eel temperature in agreement with
the bulk samples.
