We theoretically studied the length dependence of electron transport through a series of conjugated molecules, including molecular wires composed of benzene rings in parallel and oligophenylene with a single backbone. We found that the conductance of the parallel system is more than twice that of the single system, providing clear evidence for constructive quantum interference. Furthermore, the results show an exponential decay of conductance with increasing molecular length for both systems. The decay factor of the parallel system is much smaller than that of the single system. These properties can be understood from the interwire orbital overlap and the splitting of molecular orbitals in the parallel system, which significantly enhances the transmission probability of electron transport through the parallel system.