  We introduce, for each state of a bosonic quantum field, its quadrature
coherence scale (QCS), a measure of the range of its quadrature coherences.
Under coupling to a thermal bath, the purity and QCS are shown to decrease on a
time scale inversely proportional to the QCS squared. The states most fragile
to decoherence are therefore those with quadrature coherences far from the
diagonal. We further show a large QCS is difficult to measure since it induces
small scale variations in the state's Wigner function. These two observations
imply a large QCS constitutes a mark of "macroscopic coherence". Finally, we
link the QCS to optical classicality: optical classical states have a small QCS
and a large QCS implies strong optical nonclassicality.
