  We discuss the temperature dependence of the Hall conductivity $\sigma_{xy}$
in the magnetic-field-induced spin-density-wave (FISDW) state of the
quasi-one-dimensional Bechgaard salts (TMTSF)_2X. Electronic thermal
excitations across the FISDW energy gap progressively destroy the quantum Hall
effect, so $\sigma_{xy}(T)$ interpolates between the quantized value at zero
temperature and zero value at the transition temperature T_c, where FISDW
disappears. This temperature dependence is similar to that of the superfluid
density in the BCS theory of superconductivity. More precisely, it is the same
as the temperature dependence of the Fr\"ohlich condensate density of a regular
CDW/SDW. This suggests a two-fluid picture of the quantum Hall effect, where
the Hall conductivity of the condensate is quantized, but the condensate
fraction of the total electron density decreases with increasing temperature.
The theory appears to agree with the experimental results obtained by measuring
all three components of the resistivity tensor simultaneously on a
(TMTSF)_2PF_6 sample and then reconstructing the conductivity tensor.
