Using a scanning tunneling microscope, we measured high-bias conductance of single polyporphyrin molecular wires with lengths from 1.3 to 13 nm. We observed several remarkable transport characteristics, including multiple sharp conductance peaks, conductances as high as 20 nS in wires with lengths of >10 nm, and nearly length-independent conductance (attenuation <0.001 angstrom(-1)). We carried out first-principles simulations on myriad metal molecule metal junctions. The simulations revealed that the measured conductance is coherent resonant transport via a delocalized molecular orbital.