On geometrical scaling of split-ring and double-bar resonators at   optical frequencies
  In this paper we consider the resonant frequency of split-ring resonators and double-bar resonators used to create artificial magnetic response at terahertz and optical frequencies. It is known that geometrical scaling of the resonant frequency of split rings breaks down at high frequencies (in the visible) due to electromagnetic properties of metals at those frequencies. Here we will discuss this phenomenon in terms of equivalent inductance and capacitance of the ring, derive an approximate formula for the resonant frequency of split rings in the visible and show how the limiting value of the resonant frequency of extremely small split rings and double bars depends on particle shape. 
Sergei Tretyakov