  Using three-dimensional particle-in-cell simulations, we show that few-MeV
electrons can be produced by focusing tightly few-cycle radially-polarized
laser pulses in a low-density atomic gas. In particular, it is observed that
for the few-TW laser power needed to reach relativistic electron energies,
longitudinal attosecond microbunching occurs naturally, resulting in
femtosecond structures with high-contrast attosecond density modulations. The
three-dimensional particle-in-cell simulations show that in the relativistic
regime the leading pulse of these attosecond substructures survives to
propagation over extended distances, suggesting that it could be delivered to a
distant target, with the help of a properly designed transport beamline.
