With ultrashort laser pulses (25 fs) and low energy (E=15mJ), we observe laser plasma channels with a length over 5m in air. The diameter of the plasma channel is measured to be 120mm. The average electron density in the channel is inferred to be about 10 18W/cm3 with an interferometric method. At the same time, the temporal evolution of the electron density in the plasma channel is investigated. The resistivity in the plasma channel is measured to be less than 1V cm. It is suggested that the lifetime of the plasma channel can be greatly prolonged by launching another long laser pulse along the plasma channel.