We report a drastic reduction of air resistivity following the passage of a self-guided femtosecond pulse from a Ti:sapphire laser system at 800 nm with energies per pulse between 1 and 14 mJ and a pulse duration of 120 fs. Connected plasma filaments with a length that can exceed 150 cm are created by these pulses. The presence of a conducting plasma channel results from multiphoton ionization of air molecules in the filament core.