PowerShell - Unable to save Default Column Values

I have created a document library (A) in a Team Site. I have created 2 folders (B and C) inside the document library. I have added a content type to the library. The Content Type contains 2 columns rmName and rmKey (both internal names). I want to set a column level default on the column rmKey for folder B only. I tried the following code and this does not set the default:

$web = Get-SPWeb "http://abc:39325/sites/COSC"
$list = $web.Lists["A"]
$defaults = New-Object -TypeName Microsoft.Office.DocumentManagement.MetadataDefaults -ArgumentList $list
$folder = $web.GetFolder("sites/COSC/A/B")
$defaults.SetFieldDefault($folder, "rmKey", "Default Value")
$defaults.Update()

I tried replacing $folder with $list.RootFolder. This works, but it sets the default on the Library and both Folders instead of only folder B.

$web = Get-SPWeb "http://abc:39325/sites/COSC"
$list = $web.Lists["A"]
$defaults = New-Object -TypeName Microsoft.Office.DocumentManagement.MetadataDefaults -ArgumentList $list
$defaults.SetFieldDefault($list.RootFolder, "rmKey", "Default Value")
$defaults.Update()

What am I doing wrong in the first script? I feel there is something wrong in $folder. I tried using the Guid of "B" as weel as recursion to get folder but was not able to set the default value on "B".