We study a half filled ladder of spinless fermions. We show that contrarily
to a single chain, the ladder becomes a Mott insulator for arbitrarily small
repulsive interactions. We obtain the full phase diagram and physical
quantities such as the charge gap. We show that there is only a single
insulating phase for repulsive interactions, regardless of the stength of the
interchain hopping and single chain Mott gap. There is thus no
confinement-deconfinement transition in this system but a simple crossover. We
show that upon doping the system becomes a Luttinger liquid with a universal
parameter K=1/2 different from the one of the single chain (K=1/4).Comment: 9 pages, 6 figures, uses revtex4 beta4 and natbib(included);
submitted to PR