Here's why:

* Conductance is the measure of how easily an electric current flows through a material. It's the reciprocal of resistance, and its unit is Siemens (S).

* Ionic conductance specifically refers to the conductance of ions in a solution.

* The 'per meter' part (m⁻¹) accounts for the length of the solution through which the ions are moving.

Therefore, ionic conductance is expressed as Siemens per meter (S/m).