Background field for a scattering field define

Hello.

Manually we can enter a scatter field define by user like this:
Ex= 0, Ey=0 and Ez=E0*exp(-j*k0*x) with E0=1 [V/m] for example.

Strangely, for 2D cases, abs(emw.Ebx) = E0/2. (it should be equal to E0?)
and for 3D cases, abs(emw.Ebx)=E0/3. (it should be equal to E0?)

There is something that escapes me, may be someone can help me?
Xavier