A new physical effect of a plasma channel formation by the ponderomotive force of a wakefield generated by a laser pulse with a length of the order of the electron plasma wavelength is discussed. For a narrow pulse, wherein the width is less than c/ωpe ( ωpe and c are the plasma frequency and light velocity, respectively), the channel has an annular form with on-axis density maximum. The depth of the channel increases with the distance from the pulse until the phase mixing arises and the wake starts to break. The linear fluid theory is used to obtain the scaling for wave-breaking conditions. The results of numerical simulations for high intensity laser pulses are in good agreement with theoretical predictions.