Considering both the gradient decay of the real disorder and the contact scattering, we investigate the electronic transport in quasi-one-dimensional nanowires by developing a decomposition elimination method for Green's function matrix. In the presence the contact scattering, the conductance oscillates with energy. For some energies of incident electrons, an abnormal enhancement is obtained in the average conductance due to the destroyed coherence by the introduction of much low disorder, showing that there appears a new conductance peak. In the absence of disorder gradient, the average conductance firstly decreases then increases with disorder strength, indicating that there exists a localization-delocalization transition. In the presence of linearly decaying disorder, the average conductance increases slightly in a strong disorder region. In the case of the Gaussian-type decaying disorder, the average conductance decreases exponentially and the localization-delocalization transition disappears, which is different from previous thereotical result. The results are helpful for the design and the application of quasi-one-dimensional nanowires device.