It was recently claimed that in nonlinear coherent drift wave theory the temperature gradient kappa T=d ln T(x)/dx is irrelevant and that, instead of kappa T, the derivative d kappa n/dx of the density gradient kappa n=d ln n(x)/dx determines the nonlinearity. It is shown that this claim is erroneous in general and is caused by neglecting the space dependence of the phase velocity u(x) approximately=- kappa n(x)T(x) of the waves. The result is derived for quasi-one-dimensional soliton-like waves with e Phi /T<<1. Their duration is limited by the inhomogeneity of the refractive index.