  We study the magnetic-field-induced spin-density-wave (FISDW) phases in TMTSF
organic conductors in the framework of the quantized nesting model. In
agreement with recent suggestions, we find that the SDW wave-vector ${\bf Q}$
deviates from its quantized value near the transition temperature $T_c$ for all
phases with quantum numbers $N>0$. Deviations from quantization are more
pronounced at low pressure and higher $N$ and may lead to a suppression of the
first-order transitions $N+1\to N$ for $N\ge 5$. Below a critical pressure, we
find that the N=0 phase invades the entire phase diagram in accordance with
earlier experiments. We also show that at T=0, the quantization of ${\bf Q}$
and hence the Hall conductance is always exact. Our results suggest a novel
phase transition/crossover at intermediate temperatures between phases with
quantized and non-quantized ${\bf Q}$.
