  Transport and 63^Cu-NMR, Knight shift and T_1, measurements performed on the
two-leg spin ladders of Sr_2Ca_{12}Cu_{24}O_{41} single crystals show a
collapse of the gap in ladder spin excitations when superconductivity is
stabilised under a pressure of 29 kbar. These results support the prediction
made with exact diagonalisation techniques in two-leg isotropic t-J ladders of
a transition between a low-doping spin gap phase and a gapless 1-D
Tomonaga-Luttinger regime.
