Two concentric, thin conducting spherical shells have inner radius \(a\) and outer radius \(b\), respectively. The region between the shells (\(a \le r \le b\)) is filled with a material whose conductivity varies radially according to \(\sigma(r) = \sigma_0 \left(\dfrac{a}{r}\right)\), where \(\sigma_0\) is a positive constant and \(r\) is the radial distance from the center. A constant potential difference \(V_0\) is maintained between the inner and outer shells. Which of the following expressions represents the total leakage current \(I\) flowing radially between the two shells?