Using powershell make the SharePoint choice field default value to null

I have a custom list "cl1" which has a choice field "Choice". The values for the choice field are "A", "B","C" & "D".
If you create a Choice column through UI the default value will be selected as "A".

In this we will be seeing how to change the choice field default value to null using powershell

$site = Get-SPSite "http://serverName:1111/sites/sample "
$web=$site.Rootweb
$list = $web.Lists.TryGetList("cl1")
$field = $list.Fields["Choice"]
$field.DefaultValue = $null
$field.Update() 
$list.Update()