Resonance enhancement of high-order harmonic generation has recently been found in the interaction of intense ultra-short laser pulses with laser ablated plasma plumes. It is a promising route towards the production of an intense and coherent extreme ultraviolet radiation source. However, the mechanism of this resonance enhancement is still not clear. There are two possible explanations; one relies on a better recombination cross-section through an auto-ionization state in the single-atom response. The other, relies on improved phase matching
conditions around the resonance. Here we try to discriminate between these two conjectures by measuring coherence lengths of the harmonics, both on resonance and off resonance. Our findings support the single-atom response hypothesis.