does EM practically depend on length? if yes then how?

[ms_90]

I have read about blech length below which EM will not be there and this length depends on current density, but I am not getting that how it is possible because as per my knowledge for shorter lengths Imax will be more then how EM will not occur?

consider following scenario.

If one metal track is having 5um length and another is having 55um length. Same current is flowing through the both nets. Where Electro migration will be more and why?

thanks in davance