  We consider maximal violations of the Leggett-Garg inequality, obtained by
maximising over all possible measurement operators, in relation to non-unitary
aspects of the system dynamics. We model the action of an environment on a
qubit in terms of generic quantum channels and relate the maximal value of the
Leggett-Garg correlator to the channel parameters. We focus on unital channels,
and hence on decoherence. In certain important cases, exact relations between
the channel parameters and maximal violations can be found. Moreover, we
demonstrate the existence of distinct thresholds for the channel parameters,
below which no violation of the Leggett-Garg inequality can occur.
