  The emission of above-ionization-threshold harmonics results from the
recombination of two electron wavepackets moving along a "short" and a "long"
trajectory in the atomic continuum. Attosecond pulse train generation has so
far been attributed to the short trajectory, attempted to be isolated through
targeted trajectory-selective phase matching conditions. Here, we provide
experimental evidence for the contribution of both trajectories to the harmonic
emission, even under phase matching conditions unfavorable for the long
trajectory. This is finger printed in the interference modulation of the
harmonic yield as a function of the driving laser intensity. The effect is also
observable in the sidebands yield resulting from the frequency mixing of the
harmonics and the driving laser field, an effect with consequences in
cross-correlation pulse metrology approaches.
