Old units for electrical conductivity (EC)
SI units of electric resistance and electric conductance are ohm (Ω) and siemens (Si). The unit mho ("ohm" in reverse order) for conductance should no longer be used (1 mho = 1 Si). abohm and abmho are older units from the centimetre-gram-second electromagnetic system. statohm and statmho are from the centimetre-gram-second electrostatic system.
see also this forum http://landviser.us/phpBB3/viewtopic.php?f=4&t=3&p=19#p19
The question is about conductivity, not, as the above answer, conductance. The siemens in the unit of measurement for conductance, whereas the siemens per metre is the unit for conductivity.