Conical Bessel-like pulses allow control of the propagation velocity of the main intensity peak.With few-cycle pulses, this leads to a controllable shearing effect with respect to the carrier-phase oscillation and a consequent variation of the instantaneous intensity during propagation. Numerical simulations highlight how this intensity modulation directly controls the atomic dipole phase in the process of high-order harmonic generation and isolates either the long or the short electron-trajectory contributions. We identify a propagation regime in which the harmonic field takes the form of an isolated pulse of 300 as duratio
