I think you should use exactly the same approach used to determine unitary current of voltage-sensitive ion channels, except because in this case the stimulus is the ligand concentration. You have to measure the variance of the current and the average steady state current at increasing concentrations of the ligand in the same seal. When the Var-mean curve is plotted, a parabola should be obtained if the highest concentration of ligand used can reach the channel open probability one. The resolution of the parabola (its number of points) depends on how many different concentrations of the ligand you try. If the highest concentration of ligand tested can only produce low open probabilities, there is no problem, you only need to use the first part of the parabola to determine the unitary current (and so, the comductance). In this friendly tutorial there are many more details on how to use this technique:

https://www.researchgate.net/publication/11024671_Counting_channels_A_tutorial_guide_on_ion_channel_fluctuation_analysis

Thank you so much for your response, i´m reading the article you shared looking for more mothological info... Thanks again