  We study two (very) weakly-coupled Hubbard chains in the half-filled case,
and especially the situation where the intrachain Mott scale $m$ is much larger
than the bare single-electron interchain hopping $t_{\perp}$. First, we find
that the divergence of the intrachain Umklapp channel at the Mott transition
results in the complete vanishing of the single-electron interchain hopping:
This is significant of a strong confinement of coherence along the chains.
Excitations are usual charge fermionic solitons and spinon pairs of the
Heisenberg chain. Then, we show rigorously how the tunneling of spinon-pairs
produces a magnetic interchain exchange $J_{\perp}={t_{\perp}}^2/m>0$. The
result is an insulating state with spin-gapped excitations as in the
delocalized limit (i.e. for rather large interchain hoppings), where the
two-leg ladder is in the well-known D-Mott phase. Unlike for Bechgaard salts,
the confinement/deconfinement transition at absolute zero is here a simple
crossover: no metallic phase is found in undoped two-leg ladders. This
statement might be generalized for N-leg ladders with N=3,4... (but not too
large).
