  Several mechanisms are discussed which could determine the spatial coherence
of a polariton condensate confined to a one dimensional wire. The mechanisms
considered are polariton-polariton interactions, disorder scattering and
non-equilibrium occupation of finite momentum modes. For each case, the shape
of the resulting spatial coherence function g1(x) is analysed. The results are
compared with the experimental data on a polariton condensate in an acoustic
lattice from [E. A. Cerda-Mendez et al, Phys. Rev. Lett. 105, 116402 (2010)].
It is concluded that the shape of g1(x) can only be explained by
non-equilibrium effects, and that ~10 modes are occupied in the experimental
system.
