Avoid encounter from inserting buffer in a net by breaking the connection.

Hi

I need to add antenna cells at all the input ports.

I am using the script given by Cadence to add this. But the
connectivity between the input pin and the Antenna cell is broken and
a buffer is placed in the middle when i run CTS & during timing
optimization which basically equivalent to not adding the Antenna cell
at all.

The input port will see the buffer alone instead of seeing the Antenna
cell & the buffer.

Is there any way to lock the connection between the input port & the
Antenna cell. We cant lock the net because that net has to connect to
the inner logic also.
Can we lock only a portion of the net. ie from the input port till the
Antenna cell.

OR

Is there any way to instruct encounter not to insert buffer in the
specified nets.

Thanks
vivek.