Coaxial cable: how is it possible to define a conductance?
Let's say we apply a DC voltage between the internal and the external conductors on the left end of a coaxial cable and leave the right end of the cable open. 

The applied voltage will create radial field between the internal and external conductors and some leakage current will flow, along the radial lines, based on the conductance of the imperfect dielectric. Due to the leakage current, there will be a voltage drop along the length of the internal and external conductors, but, since their resistance is much much smaller than the resistance of the dielectric, this voltage drop and, therefore, the longitudinal electric field will be negligible and won't noticeably affect the radial electric field lines and the radial leakage current.

If we connect a load (say, a resistor) between the internal and external conductors on the right end of the cable, an additional, much more significant, current will flow in the internal and external conductors, which will create an additional small voltage drop along their length and, as a result, the strength of the longitudinal electric field will slightly increase.

This longitudinal electric field components, will slightly bend the radial electric field, but, assuming that most of the applied voltage will drop on the load, the radial lines still won't be significantly distorted and, therefore, the leakage current through the dielectric will still be predominantly radial.

The diagram below shows how the electric field lines (long red arrows) maintain their radial direction in the presence of the leakage current (short blue arrows) and slightly bend in the presence of the load current (long blue arrows) due to the added longitudinal electric field component (short red arrows).



In summary, the longitudinal electric field in the internal and external conductors will have some effect on the shape of the electric field between them and, as a result, the path of the leakage current through the dielectric may by slightly deviate from radial.